Add comments to `ft_corner_is_flat'.
Add comments to `ft_corner_is_flat'.

xlgames-inc/XLE,yorung/XLE,xlgames-inc/XLE,xlgames-inc/XLE,yorung/XLE,yorung/XLE,xlgames-inc/XLE,yorung/XLE,yorung/XLE,yorung/XLE,xlgames-inc/XLE,yorung/XLE,xlgames-inc/XLE,yorung/XLE,xlgames-inc/XLE,xlgames-inc/XLE